Save each new onlink IPv6 prefix and attach an expiry timer to it.
If the prefixes overlap, take the shorter prefix and write a debug
message about the event. Once the prefix is resent in a Router
Advertisement, update the timer. Add a new event for the expiring
prefix.
Add two helper functions, one for returning a prefix length given a
Router Advertisement and the other for generic prefix matching given
an IPv6 prefix and address.